I suggest to read this book : Fondation of Databases. It will be hard but
probably you understand that database is more than a set of tables.
Codd's article, i am sure that it was one of the most revolutionary paper in the history of computer science.
Course of DL (Prof Franconi)
My favourite publication on DL ( Prof Lenzerini and his group)
Course of DL (Prof Colombetti): Parte I Parte II
Material of Phd scholl on data integration and services (DASI 06)
One of the most importat gruop in the world in wich i have the opputunity to work with them:
Context -Addict